This proposal aims to understand the cellular mechanisms by which sex stero d hormones (androgens, estrogens) alter the waveshape of spikes generated by the electric organ in a species of fish with a sexually dimorphic electric organ discharge. The 7 sections described herein can be summarized as an attempt to describe the action of steroid hormones on the biophysical properties, specifically ion conductances, of this tissue. In addition, these cells will be analyzed morphologically to determine the extent of synaptic and spiking membrane as this will aid in the interpretation of the biophysical data. Also, a medullary nucleus, called the pacemaker nucleus, which drives the electric organ, will be investigated in terms of its potential role in influencing spike waveshape in the electric organ. The significance of this study is two fold. First, by understanding how steroid hormones alter the ion conductances of excitable cells, this work will fill in the "black-box" between sex steroids and behavior. It is know where sex steroids bind in the brain and that they may alter neural activit , but their mechanisms re unknown. Thus, this may be a benefit to an understanding of hormonal modulation of behavior and, perhaps, sexual dysfunction. The other, more general, point is that this work will illuminate the fundamental question of how excitability of an excitable membrane may be regulated.